The present invention relates to a novel method for forming a highly pure thin film based upon an ion beam sputtering method and to an apparatus therefor.
As the process for producing semiconductors is becoming more sophisticated in recent years, it has been strongly demanded to prevent the reaction on the interface and to prevent the diffusion of dopant, in order to obtain precise and steep dopant distributions. To satisfy this demand, it becomes necessary to grow a thin film at a low temperature and to establish technology capable of effecting the doping at a temperature as low as possible.
A temperature of not lower than 1000.degree. C. is usually required for growing an epi-layer of silicon based on the chemical vapor deposition method (CVD). At such a high temperature, the diffusion of the dopant becomes no longer negligible, as a matter of course. In order to lower the growing temperature in the CVD method, technologies have been developed such as ultra-high vacuum CVD (UHVCVD) and low-pressure CVD (LPCVD). Even relying upon these technologies, however, the growing temperature is not low enough. In order to obtain a steep dopant distribution, the growing temperature must be further lowered.
In the light-assisted CVD, the growth of epi-layer at a temperature of not higher than 200.degree. C. has been reported without, however, still being capable of suppressing lattice defect, transition, etc. to a satisfactory degree. On the other hand, a molecular beam epitaxial growing method (MBE) has been known as an excellent low-temperature growing method. This method, however, is not capable of easily depositing silicon.
An ion beam direct deposition method is capable of growing an epi-layer at a low temperature, but is not capable of easily irradiating an ion beam of low energy in very high degree of vacuum due to the limitation on the deceleration system or on the source of generating ions.
A sputtering method can be applied for forming a thin silicon film of a device grade at a low temperature. This method is advantageous from the standpoint of suppressing impurities since less gases are produced from the source of vaporization unlike that of heated vaporization. In particular, use of the ion beam sputtering method makes it possible to grow an epi-layer at a low temperature in a high degree of vacuum.
When Kr ions or Ar ions are used for sputtering in the ion beam sputtering method, however, there arises a problem in that these ions are entrapped in the thin film, particularly, in a low-temperature zone though the amounts are smaller than those of the plasma sputtering method. In other words, the problem is that gaseous elements flying into the material from ion sources from where ions are generated and ions themselves infiltrate into the thin film.
Concerning this according to the existing technology, use is made of a combination of a high-vacuum pump called differential evacuating device and a partitioning wall in plural stages between the source of ions and the chamber for forming a thin film, in order to prevent their infiltration and to highly evacuate the interior of the chamber for forming a thin film. Despite the differential evacuating device is installed, however, it is still difficult to completely prevent the entrapping of ions. Not only this, it becomes necessary to limit the diameter of the ion beam and, besides, the apparatus as a whole becomes bulky and complex.